Method for producing multilayer wiring circuit board

ABSTRACT

A method of producing a multilayer wired circuit board that can provide sufficient adhesion strength of the interface between a conductor layer and a thermosetting adhesive layer laminated, to provide improvement in connection strength between the conductor layers and thus improvement in reliability. In this method, after a thermosetting adhesive layer is formed on a first conductor layer, an opening is formed in the thermosetting adhesive layer and solder powders are charged in the opening at normal temperature. Sequentially, a second conductor layer is formed on the thermosetting adhesive layer including the opening filled with the solder powders. Thereafter, the solder powders are melted by heating, to electrically connect between the first conductor layer and the second conductor layer.

This application is a 35 U.S.C. 371 National Stage entry ofPCT/JP02/06718, filed Jul. 3, 2002, which claims priority from JapanesePatent Application No. 2001-204885, filed July 5, 2001, and JapanesePatent Application No. 2002-025864, filed Feb. 1, 2002.

TECHNICAL FIELD

The present invention relates to a method of producing a multilayerwired circuit board.

BACKGROUND ART

A multilayer wired circuit board is formed by laminating a plurality ofwired circuit boards, each having an insulating layer and a conductorlayer formed in a specific wired circuit pattern on the insulatinglayer, through an adhesive layer sandwiched therebetween. The conductorlayers are electrically connected with each other through conductivepassages extending in a thickness direction of the adhesive layersinterposed between the conductor layers.

It is typical in the method of producing this multilayer wired circuitboard that the wired circuit boards are multilayered in the laminationprocess shown in FIG. 4, for example. That is to say, a first wiredcircuit board 3 having a first insulating layer 1 and a first conductorlayer 2 formed in a specific wired circuit pattern on the insulatinglayer is prepared, first, as shown in FIG. 4(a). Then, a thermosettingadhesive layer 5 is laminated on the first conductor layer 2 of thefirst wired circuit board 3 in its B-stage state, as shown in FIG. 4(b).Then, an opening 4 is formed in the thermosetting adhesive layer 5, asshown in FIG. 4(c). Thereafter, a solder paste 6 is screen-printed onthe opening 4, as shown in FIG. 4(d), and then is melted by heating(reflow soldering) to form a solder bump 7 a, as shown in FIG. 4(e).Thereafter, a second wired circuit board 10 having a second insulatinglayer 8 and a second conductor layer 9 formed in a specific wiredcircuit pattern on the second insulating layer is positioned to beopposed to the solder bump 7 a and laminated on the thermosettingadhesive layer 5, as shown in FIG. 4(f). Then, those laminated layersare heated under pressure to cure the thermosetting adhesive layer 5, asshown in FIG. 4(g), and thereby the first wired circuit board 3 and thesecond wired circuit board 10 are adhesive bonded together.

In this lamination process, when the solder paste 6 is melted byheating, to form the solder bump 7 a, the thermosetting adhesive layer 5in the B-stage state is reactively cured to some extent. As a result ofthis, when the second wired circuit board 10 is laminated on thethermosetting adhesive layer 5 and then is heated under pressure,fluidity of the thermosetting adhesive layer 5 is reduced, so that asufficient adhesion strength of the interface between the second wiredcircuit board 10 including the second conductor layer 9 and thethermosetting adhesive layer 5 cannot be obtained. As a result of this,an interlayer strength between the first wired circuit board 3 includingthe first conductor layer 2 and the second wired circuit board 10including the second conductor layer 9 deteriorates, leading to apossible problem of electrical conductive failure between the layers.

DISCLOSURE OF THE INVENTION

It is the object of the invention to provide a method of producing amultilayer wired circuit board that can provide sufficient adhesionstrength of the interface between the conductor layer and thethermosetting adhesive layer, to provide improvement in connectionstrength between the conductor layers and thus improvement inreliability of a multilayer wired circuit board.

The method of producing a multilayer wired circuit board according tothe present invention comprises the step of forming a thermosettingadhesive layer having an opening previously formed therein on a firstconductor layer, or forming the thermosetting adhesive layer on thefirst conductor layer, followed by forming the opening in thethermosetting adhesive layer; the step of charging solder powders in theopening at temperature of 5-50° C.; the step of forming a secondconductor layer on the thermosetting adhesive layer including theopening filled with the solder powders; and the step of melting thesolder powders by heating to electrically connect between the firstconductor layer and the second conductor layer.

According to this method, after the solder powders are charged in theopening of the thermosetting adhesive layer at temperature of 5-50° C.,the second conductor layer is formed on the thermosetting adhesivelayer. Thus, the thermosetting adhesive layer and the second conductorlayer are already laminated with each other when the solder powders aremelted by heating in a later stage. Hence, even when the thermosettingadhesive layer is reactively cured further, the adhesion strength of theinterface between the thermosetting adhesive layer and the secondconductor layer increases as the curing of the thermosetting adhesivelayer proceeds. As a result of this, sufficient adhesion strength of theinterface between the second conductor layer and the thermosettingadhesive layer is obtained. Consequently, improved connection strengthbetween the first conductor layer and the second conductor layer isprovided and thus reduced interlayer conductive failure is provided,thus enabling a multilayer wired circuit board to be produced with highreliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a production process of a multilayer wired circuitboard producing method of an embodiment of the present invention:

-   -   (a) illustrates the process of preparing a first double-sided        wired circuit board;    -   (b) illustrates the process of laminating a thermosetting        adhesive layer on one of first conductor layers of the first        double-sided wired circuit board and then forming an opening in        it;    -   (c) illustrates the process of charging solder powders in the        opening of the thermosetting adhesive layer at normal        temperature;    -   (d) illustrates the process of preparing a second double-sided        wired circuit board separately;    -   (e) illustrates the process of laminating one of second        conductor layers of the second double-sided wired circuit board        on the thermosetting adhesive layer including the opening filled        with the solder powders and bonding together the first        double-sided wired circuit board and the second double-sided        wired circuit board through the thermosetting adhesive layer;        and    -   (f) illustrates the process of melting the solder powders by        heating to form the conductive passage,

FIG. 2 illustrates details of a principal part of the production processof the multilayer wired circuit board producing method shown in FIG. 1:

-   -   (a) illustrates the process of preparing the first double-sided        wired circuit board;    -   (b) illustrates the process of forming the thermosetting        adhesive layer on one of the first conductor layers of the first        double-sided wired circuit board;    -   (c) illustrates the process of forming the opening in the        thermosetting adhesive layer;    -   (d) illustrates the process of printing the solder paste        prepared by mixing the solder powders in a solvent on the        thermosetting adhesive layer through a metal mask;    -   (e) illustrates the process of removing the solvent from the        solder paste by drying;    -   (f) illustrates the process of pressuring the solder powders at        normal temperature;    -   (g) illustrates the process of peeling a separator and removing        it from the thermosetting adhesive layer, together with the        surplus solder powders overcharged in the opening;    -   (h) illustrates the process of disposing hot pressing devices at        opposite sides of the first double-sided wired circuit board and        the second double-sided wired circuit board;    -   (i) illustrates the process of pressing and/or heating the        double-sided circuit boards; and    -   (j) illustrates the process of melting the solder powders by        heating to form the conductive passage,

FIG. 3 illustrates details of a principal part of the multilayer wiredcircuit board producing method shown in FIG. 1, illustrating theprocesses as an alternative to the processes of (d) to (g) of FIG. 2,for charging the solder powders in the opening of the thermosettinglayer:

-   -   (d) illustrates the process of printing an adequate quantity of        (substantial quantity of) solder paste for the opening under        normal temperature, with the separator as the mask;    -   (e) illustrates the process of removing the solvent from the        solder paste by drying; and    -   (f) illustrates the process of peeling the separator from the        thermosetting adhesive layer, and

FIG. 4 illustrates details of a principal part of the production processof a conventional multilayer wired circuit board producing method:

-   -   (a) illustrates the process of preparing the first double-sided        wired circuit board;    -   (b) illustrates the process of laminating the thermosetting        adhesive layer on the first conductor layer of the first wired        circuit board;    -   (c) illustrates the process of forming the opening in the        thermosetting adhesive layer;    -   (d) illustrates the process of screen-printing the solder paste        on the opening;    -   (e) illustrates the process of forming the solder bump by        melting the solder paste by heating;    -   (f) illustrates the process of laminating the second conductor        layer of the second wired circuit board on the thermosetting        adhesive layer in the state of being opposed to the solder bump;        and    -   (g) illustrates the process of bonding together the first        double-sided wired circuit board and the second double-sided        wired circuit board through the thermosetting adhesive layer.

DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an embodiment of the production process of themultilayer wired circuit board producing method of the presentinvention. FIG. 2 illustrates the details of a principal part of theproduction process of the method. The illustrations of FIG. 2 aredepicted as sectional views as viewed from the direction orthogonal tothe direction from which the illustrations of FIG. 1 are viewed. In thefollowing, an embodiment of the multilayer wired circuit board producingmethod of the present invention will be described with reference toFIGS. 1 and 2.

In the first process of this method, a first double-sided wired circuitboard 13, which comprises first conductor layers 12 formed in the formof a specific wired circuit pattern on both sides of a first insulatinglayer 11, respectively, is prepared, as shown in FIG. 1(a) and FIG.2(a).

No particular limitation is imposed on the materials of the firstinsulating layer 11, as long as they are commonly used for theinsulating layer of the wired circuit board. For example, films ofsynthetic resins, such as a polyimide resin, an acrylic resin, apolyether nitrile resin, a polyether sulfonic resin, a polyester resin,a polyethylene naphthalate resin, and a polyvinyl chloride resin may beused as the insulating layer 11. Preferably, the film of the polyimideresin is used. The first insulating layer 11 is usually formed to havethickness of 9-100 μm, or preferably 9-35 μm.

No particular limitation is imposed on the materials of the firstconductor layer 12, as long as they are commonly used for the conductorlayer of the wired circuit board. For example, foils of metals, such ascopper, nickel, gold, solder or their alloys may be used for the firstconductor layers 12. Preferably, the foil of copper is used. Each of thefirst conductor layers 12 usually has thickness of 9-50 μm, orpreferably 9-25 μm. The first conductor layers 12 are formed in the formof a specific wired circuit pattern on both sides of the firstinsulating layer 11 respectively by a known patterning process, such asa subtractive process, an additive process or a semi-additive process.

The first double-sided wired circuit board 13 has first through holes 31bored in the first insulating layer 11 to extend through it in athickness direction thereof The first through holes 31 are plated withmetal, such as copper, nickel, gold, solder, or alloys thereof, to formfirst through-hole plated layers 32 for electrically connecting betweenthe first conductor layers 12 formed at both sides of the firstinsulating layer 11. The first through-hole plated layers 32 are formedto be continuous to the first conductor layers 12.

Sequentially, the thermosetting adhesive layer 15 is formed in theB-stage state on one of the first conductor layers 12 of the firstdouble-sided wired circuit board 13. The opening 14 is then formed inthe thermosetting adhesive layer 15, as shown in FIG. 1(b) and FIGS.2(b)-(c).

No particular limitation is imposed on the thermosetting adhesive 15, aslong as it can be usually used for the adhesive layer of the wiredcircuit board and can be put into the B-stage state (the state in whichthe thermosetting adhesive is cured to such an extent that the adhesivelayer can be held in its specific form). For example, acrylic adhesive,epoxy adhesive, amideimide adhesive, polyimide adhesive, and blendedadhesives thereof can be cited as the thermosetting adhesive. Thethermosetting adhesive usually has thickness of 25-100 μm, or preferably40-60 μm.

Also, the thermosetting adhesive 15 to be cured in temperature of notless than 100° C., or preferably 125-200° C., is preferably used.

The thermosetting adhesive layer 15 can be formed in the B-stage stateon the first conductor layer 12 in the following manner, for example. Asshown in FIG. 2(b), after solution containing the thermosetting adhesiveis applied to the first conductor layer 12, the solution is dried byheating so that the adhesive can be put into the B-stage state at thesame time as the solution is dried. Alternatively, an adhesive sheetcomprising thermosetting adhesive previously put in the B-stage statemay be laminated on (temporarily bonded to) the first conductor layer 12by application of heat thereto and/or by application of pressurethereon.

Also, the opening 14 may be formed in the thermosetting adhesive layer15 in the following manner. For example, the opening 14 may be formed byboring the thermosetting adhesive layer 15, for example, by using laser,such as YAG laser, as shown in FIG. 2(c). The opening 14 is formed toextend through the thermosetting adhesive layer 15 in thicknessdirection thereof at a position in the first conductor layer 12 opposedto a connecting area with a second conductor layer 23 as mentionedlater. Preferably, the opening 14 has a diameter of 50-300 μm, orpreferably 50-200 μm, when formed in a circular shape, for example.

In this process, the opening 14 may be previously formed in thethermosetting adhesive layer 15 before the thermosetting adhesive layer15 is formed on the first conductor layer 12, rather than after thethermosetting adhesive layer 15 is formed on the first conductor layer12. The thermosetting adhesive layer 15 having the opening 14 previouslyformed may be formed on the first conductor layer 12, for example, byboring the adhesive sheet comprising the thermosetting adhesive to formthe opening 14 in it with a drill, a punch or the like, first, and thenlaminating the adhesive sheet having the opening 14 on the firstconductor layer 12, to form the thermosetting adhesive layer 15. Forexample, when viewed in FIG. 2, this process could be depicted in thestep FIG. 2(a) and the step FIG. 2(c), skipping the step FIG. 2(b).

In the processes shown in FIGS. 2(b)-(c), in order to keep thethermosetting adhesive layer 15 from the solder powders 17 adhering toits unwanted locations in the solder powder 17 charging processmentioned later, a separator 16 is preferably laminated on a surface ofthe thermosetting adhesive layer 15 on its side opposite to the sidethereof contacting with the first conductor layer 12.

Synthetic resin films, such as a polyester resin film, a polyethylenenaphthalate resin film, and a polyimide resin film, may be used as theseparator 16. The separator 16 has thickness of 7.5-50 μm. The separator16 is bonded to the surface of the thermosetting adhesive layer 15before the opening 14 is formed therein and, then, the separator 16 andthe thermosetting adhesive layer 15 are both bored to form the opening14 in both of them.

Then, the solder powders 17 are charged in the opening 14 of thethermosetting adhesive layer 15 at temperature of 5-50° C., as shown inFIG. 1(c) and FIGS. 2(d)-(g).

Although no particular limitation is imposed on the solder powders 17,for example binary compositions comprising Sn/Ag, Sn/Cu, Sn/Sb andSn/Zn, and multiple compositions comprising Sn/Ag/Cu and Sn/Ag/Cu/Bi,can be cited as the solder powders that can be used. The solder powders17 having an average particle diameter of not more than 50 μm, orpreferably not more than 20 μm, are preferably used.

The solder powders 17 are charged in the opening 14 of the thermosettingadhesive layer 15 at temperature of 5-50° C. in the following manner.First, the solder powders 17 are mixed in the solvent 18 to prepare thesolder paste 19. Then, after the metal mask 20 is disposed on thethermosetting adhesive layer 15, the solder paste 19 is printed on thethermosetting adhesive layer 15 through the metal mask 20 at temperatureof 5-50° C., or preferably 10-30° C., or specifically under normaltemperature, as shown in FIG. 2(d). It should be noted that the phraseof “under normal temperature” is intended to mean “in an atmosphere ofroom temperature without heating”.

Although no particular limitation is imposed on the solvent 18, thesolvent 18 that can be removed by drying in the temperature range of 75°C. to 200° C., or preferably 75° C. to 160° C., in the next dryingprocess should preferably be selected. When the solvent that can bedried at temperature lower than 75° C. is selected, storage stabilityand continuous printability of the solder paste 19 may deteriorate. Onthe other hand, when the solvent that can be dried at temperature higherthan 200° C. is selected, the thermosetting adhesive layer 15 may becured in the drying process to such an extent that the adhesion strengthof the interface between the thermosetting adhesive layer 15 and thesecond conductor layer 23 mentioned later may deteriorate.

More specifically, the solvent 18 prepared by adding e.g. amide resin toaliphatic alcohol to provide viscosity improvement effects is preferablyused. The amount of amide resin added is in the approximate ratio of0.005-5% to solder powder 17 by volume.

The solder paste 19 can be prepared, for example, by mixing the solderpowders 17 and the solvent 18 to be in the approximate ratio of 1:0.5-2by volume.

The metal mask 20 is formed in a specific pattern corresponding to theopening 14. The metal mask 20 is disposed on the thermosetting adhesivelayer 15 and an excessive amount of solder paste 19 for the opening 14is printed on the opening 14 from above the metal mask 20.

Sequentially, the solvent 18 is removed by drying, as shown in FIG.2(e). The solvent 18 can be dried and removed, for example, by heatingit at temperature of 75-200° C., or preferably 75-160° C., as mentionedabove, for an adequate time. The drying time may be determined properly,depending on the amount of solder paste 19 to be charged in the opening14 and the size of the first double-sided wired circuit board 13.However, an excessively short drying time causes the solvent to remainin the solder paste, so that the remaining solvent may be out-gassed byheating to cause a possible conductive failure. On the other hand, anexcessively long drying time causes the thermosetting adhesive layer 15to be cured to such an extent that the adhesion strength of theinterface between the thermosetting adhesive layer 15 and the secondconductor layer 23 mentioned later may deteriorate. From this point ofview, the drying time is preferably in the range of 1 to 5 minutes, forexample.

Then, the solder powders 17 are pressed at temperature of 5-50° C., orpreferably 10-30° C., or specifically under normal temperature, as shownin FIG. 2(f). The application of pressure on the solder powders 17 canbe done by pressing a pressurizer 21, such as a press or a pressureroller, from above the solder powders 17 at 0.5-10 MPa for 1 second to 5minutes, for example. As a result of the application of pressure, thesolder powders 17 are deformed and charged in the opening 14 in highdensity. It is preferable that the pressure is applied on the solderpowders 17 so that the pressed solder powders 17 in the opening 14 canhave a volume ratio of 40-99%. When the volume ratio of the soldercharged in the opening 14 is less than 40%, the solder powders 17 is lowin adhesion. On the other hand, when the volume ratio of the solderpowders 17 charged in the opening 14 is more than 99%, the pressure ashigh as 100 MPa must be applied on the solder powders 17, so that thereis the possibility that the first double-sided wired circuit board 13may be cracked.

In this solder powders 17 charging process, all operations, except theoperation of removing the solvent 18 by drying, are performed attemperature of 5-50° C., or preferably 10-30° C., or specifically undernormal temperature.

Thereafter, the separator 16 is peeled off and removed together with thesurplus solder powders 17 flowing over the opening 14, as shown in FIG.2(g). This can allow the solder powders 17 to be charged in the opening14 in a simple and reliable manner.

In place of the processes shown in FIGS. 2(d)-(g), alternativeprocesses, such as those shown in FIGS. 3(d)-(f), may be taken to chargethe solder powders 17 in the opening 14 of the thermosetting adhesivelayer 15. Specifically, with the separator 16 as the mask, an adequatequantity of (substantial quantity of) solder paste 19 for the opening 14is printed at temperature of 5-50° C., or preferably 10-30° C., orspecifically under normal temperature, as shown in FIG. 3(d). Then,after the solvent 18 is removed by drying in the same manner as in theabove, as shown in FIG. 3(e), the separator 16 is peeled off withoutapplying any pressure on the solder powders 17 to allow the solderpowders 17 to be charged in the opening 14, as shown in FIG. 3(f).

Sequentially, in this method, a second double-sided wired circuit board24, which comprises the second conductor layers 23 formed in a specificwired circuit pattern on both sides of a second insulating layer 22,respectively, is prepared separately, as shown in FIG. 1(d). As shown inFIG. 1(e) and FIGS. 2(h)-(i), one of the second conductor layers 23 ofthe second double-sided wired circuit board 24 is laminated on thethermosetting adhesive layer 15 including the opening 14 filled with thesolder powders 17.

The same layers as the first insulating layer 11 and the first conductorlayer 12 may be used as the second insulating layer 22 and the secondconductor layer 23. In the second double-sided wired circuit board 24,in which the second conductor layers 23 are formed in a specific wiredcircuit pattern on both sides of the second insulating layer 22,respectively, as is the case of the first double-sided wired circuitboard 13, the second conductor layers 23 are electrically connected witheach other through second through-hole plated layers 34 formed in secondthrough holes 33 extending through the second insulating layer 22 in athickness direction thereof.

Then, one of the second conductor layers 23 of the second double-sidedwired circuit board 24 is positioned to be opposed to the opening 14 andis laminated on the thermosetting adhesive layer 15. The lamination canbe done, for example, by using heating/pressing devices 25, such as hotpressing devices or hot pressing rolls, which are located opposite thefirst double-sided wired circuit board 13 and the second double-sidedwired circuit board 24, respectively, as shown in FIG. 2(h), to pressand/or heat them, as shown in FIG. 2(i). The conditions for thepressuring and/or the heating may be determined properly, depending onthe size of the first double-sided wired circuit board 13 and that ofthe second double-sided wired circuit board 24. It is preferable thatthe first double-sided wired circuit board 13 and the seconddouble-sided wired circuit board 24 are pressed at pressure of 1-10 MPa,or preferably 3-5 MPa, and/or at temperature of 160-225° C., orpreferably 175-200° C., for example. By this pressuring and/or theheating, the thermosetting adhesive layer 15 which is in the B-stagestate is cured and, as a result of this, the second double-sided wiredcircuit board 24 is bonded to and laminated onto the first double-sidedwired circuit board 13 through the thermosetting adhesive layer 15.

Thereafter, as shown in FIG. 1(f) and FIG. 2(j), the solder powders 17are melted by heating to form a conductive passage 26 for electricallyconnecting between the first conductor layer 12 and the second conductorlayer 23 confronting each other. After these processes, amultiple-layered (four-layered) wired circuit board 27 is produced.

The temperature required for the solder powders 17 to be melted byheating is set to be higher than the melting temperature of the solderpowders 17 used. It is preferable that the solder powders 17 are meltednot only by the application of heat thereto but also by the applicationof pressure thereon at 1-10 MPa, or preferably 3-5 MPa.

The curing of the thermosetting adhesive layer 15 and the melting of thesolder powders 17 by heating may be taken sequentially in separateprocesses or may alternatively be taken simultaneously in the sameprocess by selecting the conditions therefor properly.

In this method, after the solder powders 17 are charged in the opening14 of the thermosetting adhesive layer 15 at temperature of 5-50° C., orpreferably 10-30° C., or specifically at normal temperature, the secondconductor layer 23 is laminated on the thermosetting adhesive layer 15.More specifically, in the solder powders 17 charging process, the secondconductor layer 23 is laminated on the thermosetting adhesive layer 15in the condition in which the curing of the thermosetting adhesive layer15 is prevented as much as possible in the heat history for the solvent18 to be removed from the solder paste 19 by drying (the heat history athigh temperature in excess of 50° C.). Thus, the thermosetting adhesivelayer 15 and the second double-sided wired circuit board 24 includingthe second conductor layer 23 are already laminated with each other whenthe solder powders 17 are melted by heating in a later stage. Hence,even when the thermosetting adhesive layer 15 is reactively curedfurther by the melting of the solder powders 17 by heating, the adhesionstrength of the interface between the thermosetting adhesive layer 15and the second double-sided wired circuit board 24 including the secondconductor layer 23 increases as the curing of the thermosetting adhesivelayer 15 proceeds. As a result of this, sufficient adhesion strength ofthe interface between the second double-sided wired circuit board 24including the second conductor layer 23 and the thermosetting adhesivelayer 15 is ensured. Consequently, improved interlayer connectionstrength between the first conductor layer 12 and the second conductorlayers 23 confronting each other is provided and thus reduced interlayerconductive failure is provided, thus enabling a multilayer wired circuitboard 27 to be produced with high reliability and in a simple andreliable manner.

Although the embodiments in which the second double-sided wired circuitboard 24 is laminated on the first double-sided wired circuit board 13has been illustrated above, no particular limitation is imposed on thenumber of wired circuit boards laminated. Also, single-sided wiredcircuit boards may be laminated, in place of the double-sided wiredcircuit boards.

EXAMPLES

While in the following, the present invention will be described infurther detail with reference to Examples and Comparative Examples, thepresent invention is not limited to any Examples and ComparativeExamples.

Example 1

The first double-sided wired circuit board, which comprises the firstconductor layers comprising copper foils having thickness of 18 μmformed in the form of a specific wired circuit pattern on both sides ofthe first insulating layer comprising polyimide resin having thicknessof 13 μm, respectively, was prepared (See FIG. 1(a) and FIG. 2(a)). Thefirst double-sided wired circuit board has the first through holes boredin the first insulating layer to extend through it in a thicknessdirection thereof. The first through holes were plated with copper toform the first through-hole plated layers for electrically connectingbetween the first conductor layers.

Sequentially, after the thermosetting adhesive layer comprising theadhesive sheet of acrylic adhesive having thickness of 50 μm waslaminated on (temporarily bonded to) one of the first conductor layersof the first double-sided wired circuit board by using a vacuum press at50° C. under pressure of 1.5 MPa (See FIG. 2(b)), the opening having thediameter of 150 μm was formed in a specified area of the thermosettingadhesive layer (where the first conductor layer and the second conductorlayer are connected with each other) by using the YAG laser (See FIG.1(b) and FIG. 2(c)).

Then, after the metal mask formed in a specific pattern corresponding tothe opening was set in place on the thermosetting adhesive layer, thesolder paste was printed on the thermosetting adhesive layer through themetal mask (See FIG. 2(d)). The solder paste used was prepared by mixingthe solder powders comprising Sn/Ag having an average particle diameterof 20 μm, alcohols solvent, and amide resin in the ratio of 50:49:1 byvolume.

Then, after the solvent was dried and removed by heating at 160° C. for5 minutes (FIG. 2(e)), the solder powders were pressed under normaltemperature at 5 MPa for 5 minutes (FIG. 2(f)) and then the surplussolder powders were removed (See FIG. 1(c) and FIG. 2(g)).

Sequentially, the second double-sided wired circuit board, whichcomprises the second conductor layers comprising copper foils havingthickness of 18 μm formed in the form of a specific wired circuitpattern on both sides of the second insulating layer comprisingpolyimide resin having thickness of 13 μm, respectively, was preparedseparately (See FIG. 1(d)). The second double-sided wired circuit boardhas the second through holes bored in the second insulating layer toextend through it in a thickness direction thereof. The second throughholes were plated with copper to form the second through-hole platedlayers for electrically connecting between the second conductor layers.

Then, after one of the second conductor layers of the seconddouble-sided wired circuit board was positioned to be opposed to theopening (FIG. 2(h)), it was heated at 200° C. under pressure of 5 MPafor 30 minutes, to be laminated on the thermosetting adhesive layer (SeeFIG. 1(e) and FIG. 2(i)).

Thereafter, the solder powders were melted by heating at 250° C. underpressure of 5 MPa for 3 minutes, to form the conductive passage forelectrically connecting between the first conductor layer and the secondconductor layer confronting each other. After these processes, thefour-layered wired circuit board was produced (See FIG. 1(f) and FIG.2(j)).

Example 2

The first double-sided wired circuit board was prepared in the sameoperations as in Example 1 (See FIG. 1(a) and FIG. 2(a)).

Sequentially, after the separator comprising polyester resin film havingthickness of 12 μm was laminated on (temporarily bonded to) one of thefirst conductor layers of the first double-sided wired circuit boardthrough the thermosetting adhesive layer comprising the adhesive sheetof acrylic adhesive having thickness of 50 μm by using the vacuum pressat 50° C. under pressure of 1.5 MPa (See FIG. 2(b)), the opening havingthe diameter of 150 μm φ was formed in specified areas of the separatorand the thermosetting adhesive layer (where the first conductor layerand the second conductor layer are connected with each other) by usingthe YAG laser (See FIG. 1(b) and FIG. 2(c)).

Sequentially, with the separator as the mask, an adequate quantity of(substantial quantity of) solder paste identical with that of Example 1for the opening was printed under normal temperature (See FIG. 3(d)).Then, after the solvent was dried and removed by heating at 160° C. for5 minutes (FIG. 3(e)), the separator was peeled off without applying anypressure on the solder powders (FIG. 3(f)), so that the solder powderswere filled in the opening.

Then, the second double-sided wired circuit board was preparedseparately in the same operations as in Example 1 (See FIG. 1(d)). Then,after one of the second conductor layers of the second double-sidedwired circuit board was positioned to be opposed to the opening (FIG.2(h)), it was heated at 200° C. under pressure of 5 MPa for 30 minutes,to be laminated on the thermosetting adhesive layer (See FIG. 1(e) andFIG. 2(i)).

Thereafter, the solder powders were melted by heating at 250° C. underpressure of 5 MPa for 3 minutes, to form the conductive passage forelectrically connecting between the first conductor layer and the secondconductor layer confronting each other. After these processes, thefour-layered wired circuit board was produced (See FIG. 1(f) and FIG.2(j)).

Comparative Example 1

The same operations as those of Example 1 were performed, whereby thefirst double-sided wired circuit board was prepared, first, and, then,after the thermosetting adhesive layer was laminated on (temporarilybonded to) one of the first conductor layers, the opening was formedtherein.

Sequentially, the solder paste containing the solder powders comprisingSn/Ag having an average particle diameter 20 μm were screen-printed onthe opening. Thereafter, the solder paste was melted by heating atultimate temperature of 250° C. by using a reflow device, to form thesolder bump. Thereafter, the residue of flux was cleaned by using anaqueous cleansing agent.

Then, the second double-sided wired circuit board was preparedseparately in the same operations as in Example 1. Then, after one ofthe second conductor layers of the second double-sided wired circuitboard was positioned to be opposed to the opening, it was heated at 200°C. under pressure of 5 MPa for 30 minutes, to be laminated on thethermosetting adhesive layer. Thereafter, the lamination was furtherheated at 250° C. under pressure of 5 MPa for 3 minutes, to obtain thefour-layered wired circuit board.

Evaluation

1) Adhesion Strength Test

The four-layered wired circuit boards of Examples 1 and 2 andComparative Example 1 were evaluated by a 90° peel test on the adhesionstrength of the thermosetting adhesive layers to the second conductorlayers. The test results are shown in TABLE 1.

TABLE 1 90° peel strength Example 1 1.2 kN/m Example 2 1.2 kN/mComparative Example 1 0.3 kN/m

It is clearly seen from Table 1 that Examples 1 and 2 are higher inadhesion strength than Comparative Example 1.

2) Solder Dip Test

The four-layered wired circuit boards of Examples 1 and 2 andComparative Example 1 were evaluated by a solder dip test (260° C., 10sec.) to measure changes in resistance. The test results are shown inTABLE 2.

TABLE 2 (Unit: Ω) Initial stage 1^(st) stage 2^(nd) stage 3^(rd) stageExample 1 5.05 5.04 5.03 5.03 Example 2 5.07 5.08 5.08 5.08 ComparativeExample 1 5.15 >1000 — —

In the four-layered wired circuit board of Comparative Example 1, theadhesion strength of the interface between the second conductor layerand the thermosetting adhesive layer was low, so that the solder flowedinto the interface therebetween to cause the electrical conductivefailure in the solder dip test. On the other hand, in the four-layeredwired circuit boards of Examples 1 and 2, improvement in adhesivestrength of the interface between the second conductor layer and thethermosetting adhesive layer was obtained, so that the resistance changewas kept in an error range of ±10% and improvement in reflow solderingwas obtained.

While the illustrative embodiments of the present invention are providedin the above description, such is for illustrative purpose only and itis not to be construed restrictively. Modification and variation of thepresent invention that will be obvious to those skilled in the art is tobe covered by the following claims.

Capabilities of Exploitation in Industry

The method of producing a multilayer wired circuit board according tothe present invention provides a useful method of producing a multilayerwired circuit board that can reduce interlayer conductive failure toprovide improved reliability.

1. A method of producing a multilayer wired circuit board, the methodcomprising: providing a wired circuit board having a first conductorlayer, said board containing at least one first opening therein; one offorming a thermosetting adhesive layer having at least one secondopening previously formed therein, on said first conductor layer, andforming the thermosetting adhesive layer on said first conductor layer,followed by forming said at least one second opening in thethermosetting adhesive layer; charging solder powders in the secondopening at a temperature of 5-50° C.; applying a second conductor layeron the thermosetting adhesive layer including the second opening filledwith the solder powders; and melting the solder powders by heating toelectrically connect between the first conductor layer and the secondconductor layer.